Changing stance is a big part of gameplay. The essential concept to master is using the correct combat position for each situation.

Generally speaking, as you go down the list accuracy will increase and movement is quieter. On the other hand, as you go up the chain of stance movement is quicker and aiming time can be reduced. This is an obvious trade off system that gives you a lot of options on the battlefield.

Ready

Please click to enlarge

Mercs can move quickly but maintain a higher degree of accuracy than running. It is not as quiet as crouching or going prone but it does hamper noise a little. It is the highest active stance that can allow your squad to aim over objects. Unless you need to cover a lot of ground, this should be the default stance to keep your mercs ready to respond...